There is known a method for hardening cutting tools by providing them with a wear-resistant coating. According to this method, tools are placed in a vacuum chamber and heated to a temperature of 400.degree. to 500.degree. C. by using steady heat sources. A titanium evaporator is switched on and nitrogen is admitted into the chamber where pressure is maintained at a level of about 1.multidot.10.sup.-3 mm of mercury (cf. L. M. Djalamanova, "Progressivnye metody sozdaniya iznosostoykykh pokrytiy na rezhushchem instrumente"/"Advanced Methods for Providing Wear-Resistant Coatings on Cutting Tools"/in a collection of papers under the general title "Technologiya proizvodstva i nauchnaya organizatsiya truda i upravleniya"/"Industrial Engineering and Management"/, No. 11, Moscow, 1979). A hard, wear-resistant film of titanium nitride is deposited on the tools, whereby their wear resistance increases several times over. However, the adhesion of the coating to the tool surface is inadequate. The coating tends to peel off, which shortens the service life of the tool.
There is known another method for hardening cutting tools of high-speed steels, which consists in vacuum deposition of a wear-resistant material on the surface of the tools. The deposition process comprises condensation and ion bombardment (cf. USSR Inventor's Certificate No. 607,659, Cl. B 23 B 27/00).
This method is carried out as follows. Cutting tools to be hardened are placed in a vacuum chamber where high vacuum is produced. A negative voltage of 1 to 2 kV is applied to the tools and vacuum-arc evaporators are turned on. The tools are heated as they are being bombarded by ions of titanium accelerated to an energy of 1 to 2 keV by the negative potential. During 3 to 5 minutes, the surface of the tools is cleaned by ions. After the ion bombardment, the potential across the cutting tools is reduced to a level of 100 to 250 V and nitrogen is admitted into the chamber at a pressure of 2.multidot.10.sup.-3 mm of mercury. A layer of titanium nitride with a microhardness of about 2,500 kg/mm.sup.2 is deposited on the surface of the tools. After 5 to 7 minutes, the pressure in the chamber is reduced to 3.multidot.10.sup.-5 mm of mercury and maintained at this level for 2 to 3 minutes. During this time a layer with a microhardness of about 700 kg/mm.sup.2 is deposited on the surface of the tools. By varying the nitrogen pressure, it is possible to obtain a coating consisting of layers with different microhardnesses. Such layers adhere very well to the tool surface and effectively withstand microshocks and vibration. They also ensure a high degree of reproducibility when the tools are resharpened.
A titanium nitride coating produced through the use of the above method is destroyed during cutting due to the heating, softening, and plastic deformation of the base metal caused by friction.
Some part of the heat resulting from the cutting process is removed with the chips, but a greater part of it is transmitted to the base metal due to a good thermal contact between the coating and the base metal, and a high thermal conduction of titanium nitride.
Ion bombardment heating of cutting tools of a great weight and size takes as long as 25 to 35 minutes. Apart from taking up so much time, this process affects the surface finish of tools and blunts their cutting edges. It may even burn the cutting edges.